1. Field of the Invention
The present invention relates to a memory device where a redundancy cell is included in. More particularly, it relates to a memory device in which a structure of the redundancy file for recording information of replacing to a redundancy cell is formed as the same as that of a normal cell, for example, a defective cell can be replaced into a redundancy cell not only at a wafer stage, but also even after a chip is stored in a package.
2. Description of the Related Art
A memory device, which uses a semiconductor, has a redundancy cell for relieving a defective cell as memory capacity increases. A DRAM used as a cache memory of a computer has a redundancy cell, and stores address information of the defective cell replaced into the redundancy cells in a fuse ROM (redundancy ROM). Then, the DRAM compares a supplied address with the address stored in the redundancy ROM, inhibits an access to a normal cell and permits to access to the redundancy cell, when both addresses are coincident.
On the other hand, a ferroelectric memory using residual polarizing action of a ferroelectric material, which is one of memory devices using a semiconductor therein, has become of major interest lately as a faster rewritable non-volatile memory as the same as a DRAM. The memory cell of FeRAM has also a simple structure formed by a selection transistor and a capacitor as the same as that of the memory cell of DRAM, and therefore, there is quite a possibility to give a large memory capacity in future. As above-described, a dielectric layer of the capacitor is made by a ferroelectric material, which is polarized by applying an electric field of one direction between electrodes of the capacitor so that residual polarization retains, even after the electric field disappears, and data is recorded. Specifically, the FeRAM is a non-volatile memory where stored data can be retained even when the power is not supplied. Moreover, it is expected that the FeRAM can be used as a non-volatile memory having a large memory capacity instead of a DRAM, because the time required for rewriting or erasing data is shorter than that of EEPROM or flash memory widely used nowadays.
Development of the FeRAM has just started, so that a device having such a large memory capacity has not been developed now. Therefore, there is no suggestion relating to a redundancy cell and the structure of replacing a defective cell into the redundancy cell. However, it can be easily estimated that the redundancy cell structure becomes a requisite structure as memory capacity increases in future, and therefore, it is required to suggest the redundancy cell and the replacing structure.
The different point between the FeRAM and the DRAM or the like will be now explained as follows. Firstly, the manufacturing process is not developed so maturely that the redundancy cell structure must be as simple circuitry structure as possible. Secondly, the defective cell detection at the FeRAM occurs not only at a wafer examination but also at a burn-in test (an acceleration test) after a memory chip is encapsulated in a package, and therefore, the defective cell must be replaceable into a redundancy cell even after encapsulating a chip into a package.
Accordingly, if a fused ROM employed in a DRAM, cut by a laser, is used as a redundancy ROM, it is required to form a memory having a different structure from an ordinary cell as a redundancy ROM in a chip. Additionally, the defective cell can be relieved only at a wafer stage, and therefore, the defective cell can not be relieved after storing a chip in a package.
Further, even DRAM, the defective cell detected after storing a chip in a package can not be relieved as long as the redundancy ROM is formed by the currently used fused ROM. Further, in a normal DRAM, it is general that a column including the defective cell is replaced into a redundancy column. In the replacing method, when some defective cells are diversely generated in a chip, there is a limit to the numbers of replaceable redundancy columns, thus that prevents from relieving the defective cells. Therefore, the relieving probability is limited.